1. Field of the Invention
The present invention relates to a propagation time detecting system for detecting a propagation time of a signal through a transmission path, or detecting a propagation time difference of a plurality of different transmission lines, and, in particular, to such a system having improved accuracy.
2. Description of the Prior Art
In a mobile communication system, it is necessary to know the current location of a moving body such as an aircraft. In order to know the current location, the aircraft or a base station transmits a signal through two, for example different signal transmission lines or paths. Detection is carried out for determining a propagation time different through the different transmission lines from which the distance between the aircraft and the base station and the current location of the aircraft are calculated.
On the other hand, a radar system can also measures a distance between the base station systen and a target, by detecting a time period from emission of a signal to arrival of the signal reflected by the target.
In the known propagation time detecting system used in a mobile communication system, a transmitting station modulates a carrier by a sequence of transmission data digitized by a clock signal into a modulated signal and transmits the modulated signal as a transmission signal through different transmission lines to a receiving station. The receiving station receives the transmission signal through the different transmission lines as different received signals and reproduces the transmission data and the clock signal as different reproduced transmission data and different reproduced clock signals. Detection is carried out for determining a difference in bit number between the different reproduced transmission data and for a time difference between the difference reproduced clock signals. The propagation time difference of the different transmission lines is calculated on the basis of the detected difference in bit number and the detected time difference.
In the radar system, the radar emits the modulated signal as an emitted signal and receives the emitted signal reflected by a target as a received signal. The radar reproduces the transmission data and the clock signal from the received signal as a reproduced sequence of transmission data and a reproduced clock signal and compares the reproduced sequence of transmission data and the reproduced clock signal with the sequence of transmission data and the clock signal to detect time differences between them as indicative of the propagation time of the transmission signal between the radar and the target. Thus, the distance from the radar to the target is calculated from the detected propagation time.
In the known propagation time detecting system, the transmission signal is subjected to any distortion during propagation through transmission lines. As a result, the reproduced clock signal has a jitter or a distortion. Accordingly, the accuracy of the detected time is limited by the jitter.